Kimberlite from Rajmahal Magmatic Province: Sr-Nd-Pb Isotopic Evidence for Kerguelen Plume Derived Magmas Anil Kumar, A

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GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 20, 2053, doi:10.1029/2003GL018462, 2003

Kimberlite from Rajmahal magmatic province: Sr-Nd-Pb isotopic evidence for Kerguelen plume derived magmas Anil Kumar, A. M. Dayal, and V. M. Padmakumari National Geophysical Research Institute, Hyderabad, India Received 20 August 2003; accepted 15 September 2003; published 24 October 2003.

[1] Previous studies showed that the Rajmahal-Sylhet- manifestations of decompression melting above a ‘hot cell’. Bengal (RSB) flood basalt province (117 ± 2 Ma) in eastern Comparison of geochemical and Sr-Nd-Pb isotopic compo- India was spatially close to the active Kerguelen hotspot sitions of Kerguelen plateau basalts with Rajmahal traps led about 118 Ma ago. Yet, it could not be unequivocally Mahoney et al. [1983] and Kent et al. [1997] to believe that correlated to this hotspot due to wide variation in isotopic the Kerguelen plume had not fed the Rajmahal magmas but compositions of both the RSB and Kerguelen plateau basalts. could have provided the heat source for their production. However, we report Sr-Nd-Pb isotopic compositions Storey et al. [1992] and Ingle et al. [2002], however, argued 87 86 ( Sr/ Sri: 0.70535 to 0.70561; eNd(T): À2.6 to À3.2; that this plume could also have furnished magmas for the 206 204 Pb/ Pbi: 17.88 to 18.07) of a co-eval (116 ± 2 Ma) Rajmahal lavas. Disagreement over the source of the RSB Group II kimberlite from this flood basalt province that is basalts is because their isotopic data is affected by variable identical to recently identified pristine Kerguelen plume amounts of contamination by a MORB like component in basalts from the Kerguelen Plateau/Archipelago and Broken Group I Rajmahal lavas and by crustal contamination in the Ridge. This suggests that the Kerguelen hotspot could indeed Group II lavas [Storey et al., 1992; Kent et al., 1997] as they be responsible for the 117 Ma magmatic activity in Eastern intruded a thick Precambrian continental crust. To overcome India. INDEX TERMS: 1025 Geochemistry: Composition of the this problem, a kimberlite intrusion, with high abundances mantle; 1040 Geochemistry: Isotopic composition/chemistry; 4825 of Nd > 250 ppm, Sr > 2200 ppm and Pb > 39 ppm, having Oceanography: Biological and Chemical: Geochemistry; 9340 temporal affinity (Ar/Ar ages between 109 and 117 Ma, Information Related to Geographic Region: Indian Ocean; 8450 Pringle et al. [1994]; Kent et al. [1998] with the RSB and Volcanology: Planetary volcanism (5480). Citation: Kumar, A., occurring to the southwest of the Rajmahal trap exposures A. M. Dayal, and V. M. Padmakumari, Kimberlite from Rajmahal (supplement-1a) has been studied for their age, Sr, Nd and magmatic province: Sr-Nd-Pb isotopic evidence for Kerguelen Pb isotopic compositions. These data are presented here and plume derived magmas, Geophys. Res. Lett., 30(20), 2053, compared with the Rajmahal basalts and the Kerguelen, doi:10.1029/2003GL018462, 2003. Crozet and Conrad hotspot lavas.

1. Introduction 2. Samples and Analytical Methods

[2] A flood basalt province (Figure 1), consisting of the [3] Kimberlite samples used are from three boreholes Rajmahal, Sylhet and Bengal (RSB) traps (117 ± 2 Ma old; CMJP 32, CMJP 33, and CMJP 51, which are within about Baksi [1995]; Kent et al. [2002]; Pande and Ray [1999]) 3 km of each other and about 10 km southwest of Jharia occurs in eastern India (covering an area of 2 Â 105 km2). town. The samples consist of euhedral olivine phenocrysts, This province is part of a widespread Early Cretaceous pseudomorphed occasionally, macrocrystic and phenocrystic magmatism along the Indian, Australian/Antarctic margins phlogopite, microphenocrystic diopside, apatite, opaques, in [Kent et al., 2002]. Episodes of such large volcanism often a groundmass of k-feldspar, phlogopite, carbonates, perov- mark the initiation of sub-continental hot spots, which skite, clinopyroxene and minor amounts of amphibole. subsequently produce chains of volcanic islands as oceanic [4] Carefully selected whole-rock fractions (matrix free plates move over them. There is disagreement over which of exotic material) were crushed to less than 200 mesh and plume was responsible for the RSB basaltic activity. dissolved in a HF+HNO3 mixture. Rb-Sr, Sm-Nd and Pb According to one view, the Kerguelen hotspot that formed isotopic analyses followed standard chemical separation and the 90E ridge and now located in the Indian Ocean mass spectrometric isotope dilution procedures [Anil Kumar (Figure 1) was responsible for this basaltic outpouring et al., 1999; Sarangi et al., 2003]. Total procedural blanks [Storey et al., 1992]. Curray and Munasinghe [1991] and during the course of these analyses were,<250 pg for Rb Subramanyam et al. [1999] suggested that the east Indian and Sr and <30 pg for Sm, Nd and Pb. magmatic province, the 85E ridge was due to the Crozet plume, currently located beneath the Crozet plateau at 3. Results and Discussion 46.2S(Figure1).Muller et al. [1993] matched the southern part of the 85E Ridge (to 10N) with the Conrad [5] Results of Sm-Nd and Rb-Sr isotopic measurements hotspot (53.4oS). Contrary to these plume links Anderson on the Jharia kimberlites are listed in Table-1 and Pb 1 et al. [1992] proposed that these Cretaceous lavas were isotopic data in Table-2 of supplement-1b . Initial Sr, Nd and Pb isotopic ratios calculated using an emplacement age

SDE 9 -- 1 SDE 9 - 2 KUMAR ET AL.: KIMBERLITE FROM RAJMAHAL MAGMATIC PROVINCE

87 86 to the Kaapvaal Group II kimberlites ( Sr/ Sri = 0.7071 to 206 204 0.7109; eNd = À6.2 to À13.4; Pb/ Pbi = 17.06 to 17.63, 206 204 except Finish, Pb/ Pbi = 17.69 to 18.24, reviewed by Mitchell [1995]). This suggests distinctly different source compositions for the East Indian occurrences.

4. Comparison of Jharia Kimberlite With RSB and Indian Ocean Hotspot Lavas

[7] In Figures 2a and 2b, the Sr vs. Nd and Sr vs. Pb isotopic data of Jharia kimberlites are compared with fields

Figure 1. Physiographic elements of the Indian Ocean showing hotspots and their tracks. Location of Rajmahal, Sylhet and Bengal magmatic province (RSBP) is also indicated. of 116 Ma (supplement-1c) are illustrated in Figures 2a and 87 86 206 204 2b, on plots of eNd vs Sr/ Sri and Pb/ Pbi vs. 87 86 Sr/ Sri respectively. The initial Sr, Nd and Pb isotopic 87 86 ratios of the kimberlite ( Sr/ Sri = 0.70535 À 0.70561; 206 204 eNd = À2.6 to À3.2 and Pb/ Pbi = 17.88 to 18.07, 207 204 208 204 Pb/ Pbi = 15.51 to 15.60, Pb/ Pbi =38.00to 38.50) exhibit restricted variation and could reflect the composition of their mantle source, because minor amounts of contamination will not modify their isotopic compositions as these rocks have extremely high concentrations of Sr, Nd and Pb (Sr = 2203 to 3149, Nd = 249 to 397 and Pb = 39 to 47 ppm) in them. Their high 100 (Mg/(Mg + Fe) 87 86 values of 60–75 and Ni contents of 215–930 ppm (author’s Figure 2. (a) eNdT vs initial Sr/ Sr and (b) initial unpublished data) support this inference. 87Sr/86Sr vs 206Pb/204Pb plot of Jharia kimberlite data [6] Our kimberlite Sr, Nd and Pb isotopic ratios are similar (circular filled dots), Cretaceous Kerguelan plateau basalts to data reported on coeval lamprophyric sills from the Jharia (KE, Storey et al., 1989; Salters et al., 1992; Frey et al., 87 86 basin ( Sr/ Sri = 0.70491 to 0.70573 and eNd = À1.5 to 2002; Kent et al., 2002; Neal et al., 2002), Pristine À3.1; initial 143Nd/144Nd values were calculated using an Kerguelen plume (PKP) basalts [Weis et al., 1993, 1998], assumed Sm/Nd ratio of 0.12 for some of these samples, Rajmahal traps (RT I & II) [Mahoney et al., 1983; Storey et Rock et al. [1992]) and for one sample each from a minette al., 1992; Baksi, 1995; Kent et al., 1997), Crozet 87 86 and a lamproite dyke ( Sr/ Sri = 0.70596, eNd = À3.1, Archipelago basalts (CR) [Mahoney et al., 1996], Conrad 206 204 87 86 Pb/ Pbi = 17.82 and Sr/ Sri = 0.70493, eNd = À2.7, Rise lavas (CO) [Borisova et al., 1996], and Southeast 206 204 Pb/ Pbi = 17.82 respectively) occurring in the Ranigunj Indian Ridge (SEIR) [Mahoney et al., 2002]. Fields for basin about 50 km to their east [Middlemost et al., 1988]. On continental crust (CC) [Taylor and McLennan, 1985] and comparison (Figures 2a and 2b) of the Sr-Nd-Pd isotopic Kaapvaal Group II kimberlites (GIIK) [Mitchell, 1995] are compositions, the Jharia kimberlites possess distinctly lower also plotted. Numbers are DSDP sites. UG = upper group, 87 86 206 204 Sr/ Sri and higher eNd and Pb/ Pbi values compared LG = lower group. KUMAR ET AL.: KIMBERLITE FROM RAJMAHAL MAGMATIC PROVINCE SDE 9 - 3

87 86 for the RSB basalts [Mahoney et al., 1983; Storey et al., compositions ( Sr/ Sr À0.7046 to 0.7048; eNd +0.3 to 1992; Baksi, 1995; Kent et al., 1997], Cretaceous Kerguelen +0.9) but 206Pb/204Pb isotopic compositions (17.90 to plateau and Broken Ridge basalts [Frey et al., 2002; Kent et 17.98; Neal et al. [2002]) again overlap the kimberlite data. al., 2002; Neal et al., 2002], Cenozoic Kerguelen Archipel- Still younger basalts (85 Ma, Pringle et al. [1997]) from ago basalts [Weis et al., 1993, 1998], Recent Crozet Archi- this region (site 747) have similar 87Sr/86Sr (0.7052 to pelago basalts [Mahoney et al., 1996], Cretaceous (80 Ma) 0.7057) and eNd (À2.1 to À3.8; Frey et al. [2002]) but their lavas of the Ob and Lena sea mounts (Conrad Rise, 206Pb/204Pb ratios (17.28 to 17.64) are appreciably lower Borisova et al. [1996]) and the Southeast Indian ridge than the Jharia kimberlite. However, Broken Ridge basalts basalts [Mahoney et al., 2002]. from sites 1141 and 1142 (94–95 Ma; Duncan [2002]) have 87 86 206 204 [8] Sr-Nd isotopic data (Figure 2a) show that the Recent identical Sr/ Sr (0.7053 to 0.7056), Pb/ Pb (17.90 to products of the Crozet plume (87Sr/86Sr = 0.7040 to 0.7041 18.02) and 207Pb/204Pb (15.59 to 15.62) with only margin- and eNd = +3.5 to +4.3, Mahoney et al. [1996]) are ally higher eNd compositions (+0.3 to + 0.7; Neal et al. dramatically different from the Jharia kimberlite. On the [2002]). Sr-Nd-Pb isotopic compositions of all the Cenozoic Sr-Pb isotope plot (Figure 2b), the Crozet lavas with Kerguelen Archipelago basalts (87Sr/86Sr = 0.7051 to 206 204 206 204 appreciably higher Pb/ Pb ratios (18.79 to 19.18, 0.7058; eNd = À0.2 to À2.9; Pb/ Pb = 18.02 to 18.27; Mahoney et al. [1996]) lie far outside the kimberlite field. 207Pb/204Pb = 15.54 to 15.58, Weis et al. [1993, 1998]) also Similarly, Sr, Nd and Pb isotopic compositions of the overlap the kimberlite data. According to Weis et al. [1993, Cretaceous Conrad Rise lavas (87Sr/86Sr = 0.7044 to 1998] these basalts were the purest representatives of the 206 204 0.7053; eNd = À0.8 to À7.7; Pb/ Pb = 17.33 to Kerguelen plume (pristine Kerguelen plume ‘PKP’ lavas), 18.28, Borisova et al. [1996]) are also different from the because of their eruption in an intraplate setting, and due to Jharia kimberlites. their remarkable similarity in Sr-Nd-Pb isotopic signature [9] Though, earlier plate reconstructions of the Indian despite diverse composition (transitional and alkali basaltic Ocean region for the Early Cretaceous by Curray and and basanitic) and age (30 to 0.1 Ma). Recently, Ingle et al. Munasinghe [1991] and Muller et al. [1993] suggested [2003] have however, assigned distinct compositions for the either Crozet or Conrad hotspots to be responsible for Kerguelen plume source during Cretaceous and Cenozoic magmatism in eastern India, the distinctly different Sr-Nd- periods. The Cretaceous Kerguelen plateau and Broken Pb isotopic compositions of the Conrad Rise and Crozet Ridge basalts characterized by primitive mantle like Sr, Nd lavas do not suggest the influence of these hot spots in and Pb isotopic compositions represent their Cretaceous producing the Jharia kimberlites. composition, whereas, the Mont Crozier basalts with mod- 206 204 [10] On the other hand coeval RSB basalts show Pb erately radiogenic Pb isotopic compositions ( Pb/ Pb = 206 204 87 86 isotopic similarity ( Pb/ Pb–17.97 to 18.03, 18.60; Sr/ Sr = 0.70501 to 0.70535; eNd = À1.78 to 207Pb/204Pb–15.52 to 15.66 and 208Pb/204Pb–38.12 to 0.18) depicted the Cenozoic signature of the Kerguelen 39.07; Kent et al. [1997]) with the Jharia kimberlite but plume. Earlier, Neal et al. [2002] were also of the opinion large variations in both 87Sr/86Sr (Group I À 0.7040 to that Broken Ridge lavas from sites 1141 and 1142 repre- 0.7053; Group II À 0.7050 to 0.7084) and eNd (Group I = sented the Cretaceous Kerguelen plume composition. There- 1.9 to 4.5; Group II = À6.7 to À0.2) isotopic compositions fore, the isotopic similarity of the Jharia kimberlites, Broken [Mahoney et al., 1983; Storey et al., 1992; Baksi, 1995; Ridge basalts, several Kerguelen Plateau basalts and also Kent et al., 1997] in the former inhibit interpretation of the PKP lavas, combined with recent plate reconstruction of the role of the kimberlite mantle source in the formation of the Indian Ocean for the Early Cretaceous period [Kent et al., RSB basalts. The Early Cretaceous southern Kerguelen 2002], and location (paleolatitudinal position) of the Ker- plateau basaltic flows from site 1136 (118–119 Ma; guelen hot spot (based on new paleomagnetic results, Duncan [2002]) also show Pb isotopic similarity Antretter et al. [2002]) close to the eastern Indian margin, (206Pb/204Pb À 17.84 to 17.99, Neal et al. [2002]) but just after 120 Ma ago, is clear evidence of their derivation 87 86 higher eNd (+5.1 to +0.5) and lower Sr/ Sr (0.7046 to from a common source in the Kerguelen plume. 0.7050) values. The slightly younger (112–110 Ma; Kent et [11] But the large variation in Sr-Nd isotopic composi- al. [2002]) basalts at sites 749 and 750 from the same region tions of the RSB lavas raise doubts about the extent of have depleted Sr, Nd and lower 206Pb/204Pb isotopic com- Kerguelen plume involvement in them. In fact, similarity of 87 86 positions ( Sr/ Sr = 0.7035 to 0.7053, eNd = +2.6 to +5.5; some of the RSB basalt samples (Group I, having depleted 206Pb/204Pb = 17.38 to 17.92, Frey et al. [2002]). Basaltic isotopic compositions) to SEIR basalts [Mahoney et al., lavas recovered at E´ lan Bank site 1137 (107–108 Ma; 2002] supports Anderson’s contention that these [Anderson Duncan [2002]) have the following 87Sr/86Sr (Upper Group et al., 1992] flood basalts were a manifestation of decom- À 0.7048 to 0.7050; Lower Group À 0.7055 to 0.7059), eNd pression melting within the depleted upper mantle. However, (Upper Group À1.0 to 0.0; Lower Group À2.8 to À1.0) and based on extensive trace element geochemistry, the Sr-Nd 206Pb/204Pb (Upper Group À 17.99 to 18.02; Lower Group isotopic variations not only in the RSB but also in some of À 17.96 to 18.02) isotopic compositions (Ingle et al. the Kerguelen plateau basalts (from sites 747, 750, 749, [2002]). Compared to Jharia kimberlites the upper group 1136, 1137, 1138) were inferred to have been caused due to of lavas have lower Sr and Nd isotopic compositions, variable assimilation of continental lithosphere into the whereas the lower group overlaps with them. Pb isotopic Kerguelen starting plume head [Frey et al., 2002; Ingle et compositions of both the upper and lower groups are similar al., 2002; Neal et al., 2002]. This inference is supported by to the Jharia kimberlite field. Tholeiitic basalts at site 1138 identical 206Pb/204Pb isotopic ratios in the RSB, Kerguelen (100–101 Ma; Duncan [2002]) in the central Kerguelen plateau basalts (sites 749, 1136, 1137 and 1138), Broken plateau have comparatively depleted Sr and Nd isotopic Ridge and Cenozoic Kerguelen Archipelago lavas (except SDE 9 - 4 KUMAR ET AL.: KIMBERLITE FROM RAJMAHAL MAGMATIC PROVINCE

Mont Crozier). The 206Pb/204Pb values in these rocks pre- Kent, R. W., M. S. Pringle, R. D. Muller, A. D. Saunders, and N. C. Ghose, served pristine Kerguelen signatures despite contamination Ar/Ar geochronology of the Rajmahal basalt, India, and their relationship to the Kerguelen plateau, J. Petrol., 43, 1141–1153, 2002. because of their similarity with the contaminant material, Mahoney, J. J., D. W. Graham, D. M. Christie, K. T. M. Johnson, L. S. Hall, whether upper mantle (SEIR basalts, avg. 206Pb/204Pb- and D. L. Vonderhaar, Between a hotspot and a cold spot: Isotopic 17.99, Mahoney et al. [2002]) or continental components variation in the Southeast Indian Ridge asthenosphere, 86E–118E, J. Petrol., 43, 1155–1176, 2002. (17.8–18.0, Frey et al. [2002]). Mahoney, J. J., J. D. Macdougall, G. W. Lugmair, and K. Gopalan, Ker- guelan hotspot source for Rajmahal Traps and Ninetyeast Ridge?, Nature, 303, 385–389, 1983. 5. Conclusions Mahoney,J.J.,W.M.White,B.G.J.Upton,C.R.Neal,andR.A. 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